Diablo II/Act IV

The Infernal Gate transports you from Mephisto's Durance beneath the Temple City of Travincal in Kurast to the upper courtyard of the Pandemonium Fortress, the last bastion of Heaven on the outer steppes of Hell: this is where other portals open, and there's a waypoint on the northeast side; your private stash is through the columns to the northwest, while the stairs down to the Outer Steppes are through the gateway in the southeast wall.

Tyrael[edit]

The Archangel Tyrael is also through the columns to the north, beside a fireplace in the northeast wall of the hall.

Introduction
It is good to see you again, hero. Mephisto's defeat is a great victory for the Light. I knew that you would eventually find your way here. The Pandemonium Fortress is the last bastion of Heaven's power before the Gates of the Burning Hells. This place has been hallowed by the blood of thousands of champions of the Light, many of whom were mortal, like yourself. Now the final battle against the Prime Evils draws near...and you must face it alone. I have been forbidden to aid you directly, save for a few bits of wisdom. For this is the hour of mortal Man's triumph... your triumph. May the Light protect you and the powers of Heaven shine upon your path...

Once he's introduced himself again, he'll tell you about the Fallen Angel.

Diablo II: Lord of Destruction

While he can resurrect any mercenary, bizarrely this is only in exchange for gold.

Gossip about Tyrael:

Deckard Cain
I have read much about the enigmatic Archangel Tyrael. He was revered in Horadrim lore both for his compassion for mortals and his unquenchable spirit. It was rumored that he went against the wishes of Heaven and gave the Horadrim the original Soulstones in order to trap Diablo and his Brothers.

Deckard Cain[edit]

Deckard Cain is also through the columns to the northwest, standing by your private stash to the southwest of Tyrael: talking to him prompts him to tell you about Hell's Forge. You need to talk to him again for a proper introduction.

Introduction
Can you believe this place? Did you ever dare to dream that you'd one day stand upon the crossroads between Heaven and Hell? This Pandemonium Fortress is truly miraculous. However, your journey is not yet over. Diablo still roams free in Hell, marshalling his demonic forces. Only when he is beaten will our world finally have peace. Hurry now... the sands of time slow for no one!

Gossip about Deckard Cain:

Tyrael
Long ago, I swore an oath to watch over the Horadrim and their descendants. As Deckard Cain is the last of their esteemed Order, I will not allow him to perish here so far from the lands of his birth. Be at ease, hero, I know that he is your friend. He shall come to no harm.

Halbu[edit]

Halbu stands to the north of the gateway in the southeast wall, in the lower courtyard at the base of the shallow stairs down from the upper. You cannot talk to him, but he trades and repairs armor and weapons (including javelins and other throwing weapons, but excluding bows and crossbows, although he still trades arrows and bolts for them). Unlike previous Acts, vendor prices are already reduced by 10%, without completing a quest.

Jamella[edit]

Jamella stands to the west of the gateway in the southeast wall, opposite Halbu in the lower courtyard at the base of the shallow stairs down from the upper. You cannot talk to her either, but she can heal you and trade in armor and weapons (including scepters, staves, wands, bows and crossbows, but excluding javelins and other throwing weapons), as well as tomes, scrolls, potions and keys. Unlike previous Acts, vendor prices are already reduced by 10%, without completing a quest.

She also allows you to gamble for unidentified magical items (they're only identified once you've paid gold for them).